Round-up is usually not recommended for stump treatment during spring and early summer. You might want to wait until late summer, cut the stumps off again and treat them with concentrated brush killer (usually about 15% active ingredient is recommended).

You could also spray the sprouts with brush killer during the growing season, but this would pose more of a risk to desirable plants growing around them.

Members of our lakeside subdivision did the cut and brush killer thing in late summer of '03. So far, so good. Nothing has regrown. You may have to recut because the wood may have to be 'open' to absorb the brush killer. i.e.,
NOT healed or scarred over. What do the directions on the can say? Also, there is usually a toll free number to call.
The manufacturers go thru hoops to get their products certified as safe to market, so they should have the answers you need. Also, your county agricultural agent will be of help too.
Pondy

you will need to recut to get a fresh surface to apply the 24D, but its better not to cut them "down to the ground". you can girdle them and apply the stump treatment to the girdled area.

From the smallest weed to the largest stump, the only correct time to apply herbicide is when the plant is actively growing, and to a surface that will absorb the solution (e.g. freshly cut wood cross section on stump).

When I cut down a tree, I apply undiluted 2,4-D immediately to the top of the freshly cut stump with a brush. If the tree is in a period of rapid growth (i.e. springtime) the stump will rapidly absorb the 2,4-D syrup, allowing several applications. Always wear nitrile or rubber gloves, as
2,4-D is much nastier than glyphosate.

Although this method is "safe", if the herbicide label does not list stump poisoning as an approved use of the product, the company CANNOT legally give advice on this particular use of their product. They will get in big trouble. You could also, so you must do it carefully.

Notice that every label starts with " It is a violation of federal law to use this product in a manner inconsistent with this label."

P.S. 2,4-D contaminated brushes can be neutralized in a (fairly dilute) solution of sodium hydroxide and water.

how and when can i kill a grape vine?

Actually 2,4-D is one of the safest herbicides as far as people. If you go back and read the scientific papers by the researcher who developed 2,4-D back in the 1960s, you find that he reports that to prove how safe it was, he ate 1/4 of a teaspoon of it every day for a couple of weeks. Can you imagine that being done and it getting into a peer reviewed scientific journal today?

If you are thinking of the Agent Orange disaster, the people damage was caused by a contaminant dioxin that Dupont accidently added to Agent Orange, and not the 2,4-D.